One method of improving the efficiency of a manufacturing assembly line is to increase the speed at which each of the steps are undertaken such that an existing assembly line will produce more products without requiring more space. Where the final step of an assembly line is to stack and package a number of finished products that are discharged from a rapidly moving conveyor, the individual products must be decelerated and brought to a stop without causing damage to the product. In the manufacture of a product that has a generally planar configuration, as for example a roofing shingle or a vinyl floor tile, the moving product is directed to a catcher that has at its far end a transverse panel or backstop, which absorbs some of the kinetic energy of the moving item causing the moving product to come to a stop and fall on the catcher where the remainder of the kinetic energy is dissipated thrashing around within the catcher's walls. The catcher has a horizontal surface provided by a pair of parallel rotatable star wheels. Once the product stops moving, the star wheels are rotated through ninety degrees thereby presenting a new pair of surfaces to receive the next moving product while dropping the stationary product below for stacking and packaging.
To accommodate an increase in the number of products coming out the conveyor from a production line, a diverter can be inserted in the line that diverts every other one of the completed products to a second catcher. However, providing a second catcher and packaging station significantly increases the space requirement of a production line.
Increasing the speed of an assembly line results in a higher rate of discharge of product against the backstop. Since kinetic energy varies as the square of velocity, the risk of damage increases exponentially as the discharge speed of the conveyor is increased. Furthermore, increasing the discharge speed of a conveyor reduces the amount of time that occurs between the successive products contacting a backstop.
Existing mechanical backstops are metal plates that are sometime mounted on compressive material. When a mechanical backstop is struck by a rapidly moving item, the surface of the backstop will vibrate for a short period of time as some of the energy of the impact is dissipated. However, very little of the kinetic energy of the moving product is absorbed and the remainder is dissipated bouncing the product around the walls of the catcher before coming to rest on the fins of the star wheel. When product speed is increased, the settling time, that is the time between the impact of the product against an end wall of the back stop and the product coming to rest on the star wheels, will also increase. Shortening the time between the impacts of two successive items of product can result in inadequate recovery time for a compressive backstop, damage to the product, and a longer settling time. On the other hand, shortening the time between impacts will reduce the dwell times, that is the time available between the impact against the backstop and the indexing of the star wheels.
To stop a more rapidly moving product within a shorter dwell time without increasing the risk of damage, the forces applied by a backstop to the product must be carefully controlled. The product must also be decelerated across a greater distance, which means that following the impact, the contact surface of the backstop must withdraw across a longer distance. After the contact surface has retracted and allowed a moving product to reach a stop, the contact surface must again return to its extended position prior to receiving the next moving product.
There is therefore a need for an improved backstop that is capable of decelerating a more rapidly moving item of product without causing damage to the product. Such a backstop must retract across a longer distance so as to control the forces applied to the product, and yet return to an at rest extended position in a shorter period of time to be ready to receive the next moving product.